DE69119356T2 - HIGH DENSITY AMPLIFICATION POINTING MAGNETORESISTIVE PLAYBACK HEAD WITH SHORT-CLOSED DOUBLE ELEMENT - Google Patents

Description

This invention relates to a magnetoresistive reproducing head and, more particularly, to a dual-element magnetoresistive head.

The magnetoresistive (MR) playback head has gained wide acceptance in the magnetic recording field since it was disclosed in US-A-3,493,694. The magnetoresistive head is characterized by high performance and low noise, which makes it particularly attractive for the playback of short wavelength signals. It can be manufactured by thin film vapor deposition techniques which allow relatively inexpensive production of multitrack heads with narrow magnetic track widths for high density recording applications. As such, a variety of shielded and unshielded arrangements are known, using single and dual magnetoresistive elements and incorporating a number of biasing techniques.

Dual element magnetoresistive magnetic heads are disclosed in US-A-3 860 965 and US-A-4 878 140. The magnetic heads disclosed in these patents comprise parallel magnetoresistive elements separated by thin electrically non-conductive layers. It has long been known that magnetoresistive structures such as the above, whose elements are separated by thin electrically non-conductive spacers, are subject to short-circuiting problems. Such short-circuiting can occur as a result of a pinhole in the non-conductive Spacers or can occur during lapping of the magnetic head or during operation of the magnetic head when the abrasive magnetic tape during playback can foul the soft magnetoresistive element above the spacer by creating a conductive connection with the adjacent conductive material. This has occurred, for example, in magnetic heads which use a soft adjacent layer which is magnetically biased, where a magnetoresistive element is separated by a thin electrically non-conductive spacer from a conductive magnetic material whose magnetic field induces the bias in the magnetoresistive element. US-A-4 024 489 teaches overcoming the problem by intentionally conductively connecting the magnetoresistive sensing layer and the bias layer by using a very thin (220 x 10-10 m) adjacent conductive separating layer. However, it is found that the structure disclosed in US-A-4 024 489 results in an attenuation of the signal from the single magnetoresistive transducer of 30% for a given power dissipation in the magnetic head because the current flowing through the conductive shunt layer makes no contribution to the signal output.

EP-A-0-063 397 describes a magnetic transducer having two magnetostatically coupled parallel layers of a magnetoresistive material. The layers are spaced apart by a small amount and means are used to supply a drive current to the layers simultaneously in operation. The current creates a magnetic bias of each layer so that the magnetizations of these layers are magnetically biased in opposite directions. The signal output means includes means for sensing the voltage across the parallel resistance of the layers.

Document EP-A-0 101 825 discloses a magnetoresistive transducer for reading data stored in a magnetic recording by vertical recording. The magnetoresistive transducer has two magnetoresistive transducers which generate an electrical output signal which is proportional to the difference in the vertical component of the field at the two transducer locations. The differential signal addition is built into the two terminals of this transducer so that the change in resistance between the terminals with a vertical transition arranged between the two transducer strips is at a maximum. The corresponding output voltage is single-ended, i.e. it is alternately positive and negative for locally successive vertical transitions.

In a preferred embodiment, the dual element magnetoresistive magnetic head of the present invention solves the short circuit problem between the two magnetoresistive elements (both of which serve as sensing elements and mutually magnetically biasing elements) without incurring the penalty of signal reduction due to the shunting of the sensing current.

The two identical magnetoresistive elements are neither completely isolated from each other nor conductively connected along their entire lengths by an adjacent conductive spacer, but are separated by an insulating layer having short-circuiting stubs at its ends to electrically connect the magnetoresistive elements. A current applied to the conductively connected elements splits into two equal currents which flow through the substantially identical magnetoresistive elements in the same direction to produce the bias magnetization and which serve as sensing currents for detecting the change in resistance of an element. The spacer does not Sensing current is placed in the shunt. If a short circuit occurs in the spacer insulation, no current will flow through the short circuit between the matched magnetoresistive elements because it flows in the same direction in each element and each element is matched to have the same resistance. There is no voltage difference across the short circuit and therefore the short circuits between the magnetoresistive elements do not interfere with detection of the recorded signals.

Furthermore, in the practice of the invention, the magnetoresistive elements are magnetically biased to operate in a magnetically unsaturated mode. This amounts to a "primary input" of short wavelength signals which effectively amplifies the reproduced signal over a broad range of the signal spectrum. The design criterion for determining the amplified portion of the spectrum requires a linear spacing between the magnetoresistive elements in the range of half to one-half the linear spacing between the magnetic flux changes recorded on the signal medium. Over this range of spacer thickness, the amplified sensitivity is relatively insensitive to the separation. For a short wavelength magnetic flux density of 80*10³ flux changes per inch, the spacer is typically selected to have a thickness of about 1500*10⁻¹⁰m.

The invention is described with reference to the figures.

The invention is explained in more detail below using an embodiment shown in the drawing.

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Fig. 1 is a drawing of a magnetoresistive double magnetic head according to the invention in contact with a recorded magnetic tape;

Fig. 2 is a schematic drawing of the resistances of the magnetoresistive elements of the magnetic head of Fig. 1;

Fig. 3a shows the mutual magnetic biasing of the magnetoresistive elements of the magnetic head of the invention;

Fig. 3b is a curve of the partial change in resistance of the oppositely magnetically biased magnetoresistive elements as a function of the applied magnetic field;

Fig. 4 is a graph of the partial variation of the resistance of the magnetoresistive elements of the magnetic head of the invention for an applied signal;

Fig. 5a, 5b, 5c show the signal amplification effect shown by the magnetic head of the invention;

Fig. 6 is a curve of the output signal level as a function of the recorded magnetic flux density for the magnetic head of the invention compared to that of an unshielded magnetoresistive magnetic head, and

Fig. 7 is a drawing of a second embodiment of the magnetic head of the invention.

In a preferred embodiment of the invention, the magnetoresistive elements are electrically connected to one another at their ends in the longitudinal direction, but are separated from the rest of their lengths by an electrically non-conductive spacer. With reference to Fig. 1 The magnetoresistive double magnetic head 70 consists of two magnetically, electrically and geometrically matched magnetoresistive elements 72, 74. The magnetoresistive elements 72, 74 are separated by a non-conductive spacer 76 over essentially their entire lengths. At the ends of the spacer 76 in the longitudinal direction there are two electrically short-circuiting stubs 78, 80 of the same width as the spacer 76, which conductively connect the two magnetoresistive elements to one another. A sensing and bias current 82 flowing to the magnetic head 70 via the lead wires 84, 86 is split between the two magnetoresistive elements 72, 74 into the same currents 88, 90 because the magnetoresistive elements 72, 74 are identical and because of the electrically shorting stubs 78, 80. Referring to Fig. 2, the equivalent electrical circuit of the magnetoresistive head 70 arrangement can be seen with the current 82 flowing into the parallel resistor R₇₂, which is the resistance of the magnetoresistive element 72, and into the resistor R₇₄, which is that of the magnetoresistive element 74.

It is assumed that an accidental electrical short circuit 92 occurs between the magnetoresistive elements 72, 74, for example due to a pinhole in the non-conductive spacer 76. Because of the substantially identical characteristics of the magnetoresistive elements 72, 74 and the equality of the currents 88, 90, the voltages e₁, e₂ are equal along the length of each of the magnetoresistive elements 72, 74. That is, no current flows in the electrically conductive connection 92 and the distribution of currents in the magnetoresistive elements is unchanged by the electrically conductive connection 92. Therefore, the bias and the efficiency of the signal, which are functions of the sense currents 88, 90, are during operation against the presence of short circuits.

The thin film magnetoresistive elements 72, 74 have a rectangular shape. This design results in an anisotropic shape of the magnetoresistive layer that is located along the longitudinal axis of the layer and that is also in the direction of the unbiased magnetization of the layer. As will be explained below, the bias voltage flips the magnetization from this axial direction and the signals from the medium further modulate the position of the magnetization by changing the sheet resistance. It should be noted that the signal fields from the tape 30 are directed along the short dimensions of the magnetoresistive elements 72, 74. The width of the sandwich in the longitudinal direction is typically equal to or slightly less than a track width of the data recorded on the tape 30. The values of the parameters of the magnetic head are determined by the application. For example, the appropriate parameters in a magnetic head operating with a track width of 50 micrometers and with 80*10³ changes in magnetic flux per inch are:

The widths of the magnetoresistive elements 72, 74 and the spacer 76 are equal to 50 micrometers, the thicknesses of the magnetoresistive elements 72, 74 are approximately equal to 250 * 10⁻¹⁰ m, the thickness of the spacer 76 is equal to 1 500 * 10⁻¹⁰ m and the heights of the magnetoresistive elements 72, 74 and the spacer 76 are equal to 5 micrometers.

The magnetoresistive head 70 is then seen in contact with a magnetic tape 30 having, over its entire length, alternately magnetized sections 32, 34, 36, 38, 40 containing the information recorded on the tape 30. A wavelength of the information recorded on the magnetic tape 30 signal comprises two adjacent, oppositely directed magnetized regions, for example the regions covered by arrows 32 and 34. The number of alternately magnetized sections 32, 34, 36, 38, 40 per inch is the number of changes in the magnetic flux recorded on the tape 30 per inch.

Referring to Fig. 3a, the currents 88, 90 flowing in the same direction in the magnetoresistive elements 72, 74 produce those magnetic fields which result in the mutual biasing of the elements 72, 74 because each of the magnetoresistive elements, which are also field sensing elements, acts like a soft, adjacent, magnetically biasing layer on the other. When the elements 72, 74 are magnetically and geometrically equal and because the amplitudes of the currents 88, 90 are the same, the bias field HB at the element 72 due to the action of the soft, adjacent, magnetically biasing layer of the magnetoresistive element 74 is equal in magnitude and opposite in sign to the bias field -HB at the element 74 due to the soft, adjacent biasing action of the magnetoresistive element 72. As is well known, when the magnetoresistive elements 72, 74 are magnetically biased, the magnetic field HB reverses the magnetization of the magnetoresistive element 72 in one direction and the -HB field reverses the magnetization of the magnetoresistive element 74 in the opposite direction by an equal amount.

Referring to Fig. 3b, the curves 42, 42 represent symmetrical resistance versus magnetic field variation curves for the magnetically biased magnetoresistive elements 72, 74. As described above, the bias fields in the elements 72, 74 have equal amplitudes but opposite signs, and the elements 72, 74 themselves are magnetically matched. Therefore, the curves 42, 42 (which are arbitrarily assigned to the magnetoresistive elements 72 and 74 respectively) are essentially identical and symmetrically shifted from the original by the applied bias fields. The horizontal axis of Fig. 3b is the applied signal field HS, and it will be seen that in the absence of an applied signal field the stationary bias point 44 is symmetrically arranged on the oppositely sloping sides of the curves 42, 42.

US-A-4 833 560, assigned to the same assignee as the present application, teaches aligning the induced anisotropic fields of a magnetoresistive element and an adjacent magnetic biasing layer so that their induced anisotropic fields lie in the same directions along the short dimension of the rectangular magnetoresistive elements. In the present invention, it will be recalled that each of the magnetoresistive elements 72, 74, in addition to its role as a signal detector, acts as a soft adjacent layer for magnetically biasing the other element. The induced anisotropic fields of the magnetoresistive elements of the present invention can be made to lie in the direction of the bias fields on the magnetoresistive elements, i.e., along the short dimensions of the magnetoresistive elements 72, 74, as taught by US-A-4,833,560.

Referring again to Fig. 1, it can be seen that the wavelength shown recorded on the tape 30 is such that half the wavelength is equal to a separation between the magnetoresistive elements 72, 74. Under this condition, the magnetic fields from the tape 30 to the elements 72, 74 are 180º out of phase. In Fig. 4, the variation in resistance with respect to the magnetic field curves 42, 42 is again shown together with a signal field 48 applied to the magnetoresistive element 72 (to which the curve 42 refers) while the signal field 50 is applied to the magnetoresistive element 74 (to which the curve 42 refers). The wavelength of the signals 48, 50 is equal to twice the separation of the magnetoresistive elements 72, 74 and therefore the signals 48, 50 are 180º out of phase. The signals 48, 50 move the resistance of the magnetoresistive elements 72, 74 about the bias points 44, the change in resistance for the magnetoresistive element 72 being shown in waveform 52 while that for the magnetoresistive element 74 is shown in waveform 54. It will be appreciated that the output signals derived from the above changes in resistance in the two magnetoresistive elements electrically connected in parallel are in phase and therefore the resulting voltages of the output signal due to the sense currents 88, 90 are also in phase.

The function of the invention to effect the amplification of a short wavelength reproduced signal can be understood by reference to Figures 5a, 5b and 5c which show the portions recorded by the magnetic field of the recorded medium, the induced magnetization in the magnetoresistive elements and the induced fields in the magnetoresistive elements. (The results shown in these figures are actually simultaneous and all fields are present at the same time. For clarity they are shown in the figures as occurring sequentially.) In Figure 5a there is illustrated a portion of the magnetized medium 30 which is under the magnetoresistive element 72 and the magnetoresistive element 74. The positive magnetization 36 (arbitrarily chosen to point to the left in Figure 5a) and negative magnetization 34 (in the opposite direction) recorded in the medium 30 cause an increase in the signal field HS. Figure 5a illustrates the condition where the distance between the transitions from positive magnetization to negative magnetization in the medium 30 is approximately equal to the separation distance between the magnetoresistive element 72 and the magnetoresistive element 74. This is the condition for signal amplification; however, as previously noted, the response to the thickness of the spacer is relatively insensitive when it is in the range of half to full distance between the transitions. As shown in Fig. 5a, a portion of the signal field HS intersects the lower path of magnetoresistance through the magnetoresistive elements 72 and 74. Not shown in Fig. 5a but present and necessary for operation of the device are the bias-related static fields discussed previously. The field HS shown in Fig. 5a is a dynamically increasing field due to magnetization in the medium. As it traverses the magnetically soft material containing the magnetoresistive elements, the HS field induces a magnetization M₇₂ in the magnetoresistive element 72 and a magnetization M₇₄ in the magnetoresistive element 74. The induced magnetizations M₇₂ and M₇₄ are also increasing since they result from the signal field HS. Because both the magnetoresistive element 72 and the magnetoresistive element 74 operate on the linear portions of their magnetization curves, it is correctly estimated that the magnitude of the induced magnetizations M₇₂ and M₇₄ is directly proportional to the strength of the field H₅.

Referring now to Figure 5b, the magnetization M74 induced in the magnetoresistive element 74 by the signal field H5 of Figure 5a is shown, but the field lines of the generating field HS are omitted for clarity. The induced magnetization M74 of the magnetoresistive element 74 in Figure 5b causes an increase in the field H74. The magnetic lines of force from the field H74 extend to and are captured by the magnetoresistive element 72. The captured magnetic lines of force from the field H74 induce an additional magnetization M72 in the magnetoresistive element 72. It is correctly appreciated that the direction of the field H74 on the magnetoresistive element 72 is downward, and referring again to Fig. 5a, it will be seen that the field H74 is associated with the direction of amplifying the field HS which originally caused the field H74 to increase. Thus, the field H74 further modulates the angular position of the magnetization vector of the magnetoresistive element 72 and also changes the magnetic resistance change of the magnetoresistive element 72. Referring to Fig. 5c, the induced magnetization M72 of the magnetoresistive element 72 also results in a field H72, the magnetic lines of force of which are in turn captured by the magnetoresistive element 74. The field H72 extends upward on the magnetoresistive element 74, and again referring to Figure 5b, it is noted that the field H72 amplifies the magnetization M74, which further amplifies the field H74. This "primary input" action between the two magnetoresistive elements and the signal from the medium produces an amplified output signal from the magnetoresistive elements at a given intensity of the magnetization of the medium.

When the medium moves by half the wavelength, i.e. the distance of a change in the magnetic signal flux relative to the magnetic head, the magnetization in the medium under the magnetoresistive element 72 and under the magnetoresistive element 74 has signs opposite to those described above. It will be appreciated that as a result the directions of all induced fields and the magnetizations also change signs and the overall effect is a continued amplification of the signal field as described above. The "original input" in turn enhances the effect of the signal field HS on the magnetoresistive element 72 and thus amplification takes place for both signs of the changing signal magnetic field of the medium.

Referring to Figure 6, curve 100 represents a plot of the sensitivity of an electrically connected dual magnetoresistive head according to the invention, and in comparison curve 102 shows the corresponding sensitivity of an unshielded single magnetoresistive head. It is well known that the magnetic head of curve 102 consists of a magnetoresistive element which is magnetically biased by an external, fixed bias source such as a permanent magnet. A comparison of curves 100, 102 shows the improvement achieved at short wavelengths with a magnetoresistive electrically connected dual magnetoresistive head.

As previously described, the gain increases at shorter wavelengths when the separation of the magnetic flux changes on the medium is of the same order or size as the distance between the magnetoresistive elements. As the distance between the magnetic flux transitions increases in length, the sensitivity of the magnetic head of the invention decreases slightly, with a drop in amplitudes occurring as the The length of the magnetic flux becomes so long that both of the magnetoresistive elements from the medium "recognize" a signal of the same polarity at the same time.

A second embodiment is illustrated in Fig. 7, wherein a dual element magnetoresistive playback head 10 has scanning and mutually magnetically biasing magnetoresistive elements 12, 14 that are matched according to magnetoresistive characteristics, electrical power resistance and geometric shape and dimensions. The elements 12, 14 are connected in a sandwich arrangement with an electrically conductive, non-magnetic spacer 16 between the elements 12, 14. A current 22, which is the scanning current and also the excitation current for biasing the elements 12, 14, flows in the two leads 18, 20 connected to the sandwich arrangement.

The parts of the sandwich arrangement are in electrical contact over their entire lengths and will therefore distribute any current flow in the sandwich arrangement in dependence on their relative resistances. Because the magnetoresistive elements 12, 14 are matched in electrical characteristics (as well as magnetic characteristics) and because of the symmetry of the sandwich arrangement, the current 22 will split into the partial currents 24, 26, 28, the currents 24, 26 flowing in the same direction through the magnetoresistive elements 12, 14 being of equal magnitude and the remainder of the current 22, i.e. the current 28, flowing into the spacer 16.

In this embodiment, the short circuit problem is prevented by the presence of the conductive spacer 28. In comparison to the magnetic head 70 of Fig. 1, the current shunted by the spacer 16 provides 28 does not contribute to signal detection and because of uniform power dissipation, magnetic head 10 is not as efficient as magnetic head 70. Magnetic head 10 exhibits gain characteristics similar to those shown in Fig. 6 for magnetic head 70. As can be seen in Fig. 7, the signal from the tape is also applied in the direction of the short dimension of the magnetoresistive elements 12, 14 and it is therefore advantageous to align the induced convenient axis along this dimension as previously described for magnetic head 70.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that changes and modifications can be effected within the scope of the invention.

Claims (8)

1. Magnetic head arrangement (70) for detecting magnetically recorded signals with a) a first rectangular magnetoresistive thin film element (72), b) a second rectangular magnetoresistive thin film element (74), c) a non-magnetic thin-film spacer (76) that lies flat against the magnetoresistive elements (72, 74) to separate the first (72) magnetoresistive element from the second (74) element, the spacer (76) having an electrically insulating, flat central portion and electrically conductive regions (78, 80) at its end portions, and the electrically conductive regions (78, 80) of the spacer (76) establishing an electrically conductive connection between the first (72) and second (74) magnetoresistive elements, d) means for a simultaneous current flow (84, 86) in the first (72) and second (74) elements, whereby the elements magnetically bias each other to the same extent in opposite directions, e) wherein the first (72) and second (74) magnetoresistive elements are arranged such that a magnetic signal field is coupled via a magnetically recorded signal along the short dimensions of the magnetoresistive elements (72, 74), characterized in that separation of the magnetoresistive elements (72, 74) causes a change in resistance for one magnetoresistive element and becomes in phase with a change in resistance of the other magnetoresistive element, thereby increasing the output signal voltage which is dependent on the current flow resulting in longitudinal sensing in the first (72) and second (74) magnetoresistive elements. 2. Magnetic head arrangement according to claim 1, characterized in that the first and second magnetoresistive elements are premagnetized in an unsaturated manner. 3. Magnetic head arrangement according to claim 2, characterized in that the first and second magnetoresistive elements are substantially matched according to magnetic, electrical and geometric characteristics. 4. Magnetic head assembly according to claim 3, characterized in that the first and second magnetoresistive elements each have a first and second induced anisotropic axis along the short dimensions. 5. Magnetic head arrangement according to claim 3, characterized in that the current means are means which cause currents of equal amplitude to flow in the first and second magnetoresistive elements. 6. Magnetic head assembly according to claim 5, characterized in that the means for coupling a signal field comprises means for simultaneously coupling a signal field of alternating direction to the first and second magnetoresistive elements, respectively, such alternating field cooperating with the first and second magnetoresistive elements to provide means for amplifying a magnetically recorded signal reproduced by the magnetoresistive magnetic head assembly. 7. Magnetic head arrangement according to claim 6, characterized in that the means for amplification are means for spatially separating the first and second magnetoresistive elements by an amount in the range of half to the entire spatial distance between the alternating signal fields. 8. Magnetic head arrangement (10) for detecting magnetically recorded signals with a) a first rectangular magnetoresistive thin-film element (12), b) a non-magnetic, electrically conductive, rectangular thin-film spacer (16) which lies flat against the first magnetoresistive element (12), c) a second rectangular magnetoresistive thin film element (14) abutting the conductive spacer (16), the spacer separating the first (12) and second (14) magnetoresistive elements, d) means for a simultaneous current flow (18, 20) in the first (12) and second (14) magnetoresistive elements, wherein the magnetoresistive elements (12, 14) magnetically bias each other to the same extent in opposite directions, e) wherein the first and second magnetoresistive elements (12, 14) are arranged to couple a magnetic signal field which is guided via a magnetically recorded signal along the short dimensions of the magnetoresistive elements (12, 14), characterized in that the separation of the magnetoresistive elements (12, 14) causes a change in the resistance for a magnetoresistive element and with a Change in resistance of the other magnetoresistive element becomes in phase, thereby increasing the output signal voltage which is dependent on the current flow resulting for sensing in the first (12) and second (14) magnetoresistive elements in the longitudinal direction.